1. Field of the Invention
The present invention relates to an optical amplifier control unit, an optical amplification apparatus, and a transmission system that are employed in optical communication or optical information processing. More particularly, the present invention is concerned with an optical amplifier control unit, an optical amplification apparatus, and a transmission system capable of maintaining the flatness of an amplification characteristic despite a change in ambient temperature.
2. Description of Related Art
For long-distance optical communication systems, a technique according to which an optical amplifier is inserted in an optical fiber transmission line is beginning to be generally adopted as one of means for extending a transmission distance. The optical amplifier is widely used to compensate for a distribution loss occurring in a light distribution system for distributing a light signal to various places.
In the past, an optical fiber amplifier that uses a rare-earth doped optical fiber as a gain medium or a semiconductor optical amplifier that utilizes the stimu ated emission phenomenon occurring in a semiconductor has been known as optical amplifiers for directly amplifying a light signal. Currently, the optical fiber amplifier is generally adopted as commercially available optical amplifiers.
For adapting the optical amplifiers to optical transmission systems or the like, it is necessary to retain a light signal at a certain level at any points in the system. This is intended to stabilize a transmission characteristic. Consequently, a facility is required for retaining a gain, which is produced by an optical amplifier inserted in a transmission line, at a certain level.
For stabilizing a gain to be produced by an optical amplifier, a configuration for detecting the intensity of a light signal input to the optical amplifier and that of a light signal output therefrom has been employed in the past. The pumping power supplied to the optical amplifier is adjusted so that the ratio of the power of one light signal to that of the other light signal will be set to a predetermined value. Owing to the configuration, the gain to be produced by the optical amplifier is set to a nearly constant value.
For increasing the transmission capacity of an optical transmission system, a wavelength-division multiplexed transmission technology has been discussed in earnest. In wavelength-division multiplexed transmission, a difference in power between components of different wavelengths constituting a wavelength-division multiplexed light signal is requested not to be very large. When the power difference is large, a signal-to-noise ratio to be detected relative to a feeble wavelength component is degraded markedly, and a transmission error is likely to occur. Especially, when a plurality of optical amplifiers is used to amplify and repeat a light signal in multiple stages, the optical amplifiers should preferably exhibit as flat a gain characteristic as possible relative to the wavelength components of the light signal to be amplified.
However, for a light signal of a wavelength of 1580 nm, an optical amplification apparatus becomes quite temperature-dependent. The flatness in the gain characteristic of the apparatus may largely deteriorate with a temperature change. It is therefore requested to take measures for maintaining the flatness.
An object of the present invention is to provide an optical amplifier control unit, an optical amplification apparatus, and a transmission system in which the flatness in a gain characteristics relative to a wavelength will not deteriorate despite a change in temperature.
An optical amplifier control unit in accordance with the present invention comprises a first photo-detector, a second photo-detector, a temperature sensor, and an automatic gain control circuit. The first photo-detector detects the power of light input to an optical amplification apparatus that is an object of control. The second photo-detector detects the power of light output from the optical amplification apparatus. The temperature sensor detects an ambient temperature of the optical amplification apparatus, and outputs the results of detection in the form of a temperature detection signal. Outputs of the first and second photo-detectors are fed to the automatic gain control circuit. The automatic gain control circuit produces a control signal used to adjust the strength of pumping performed in the optical amplification apparatus according to the temperature detection signal so that the ratio of the power of the input light to that of the output light will remain constant.
Another optical amplifier control unit in accordance with the present invention comprises a light signal detector and an automatic gain control circuit. The light signal detector detects the maximum or minimum power from among the powers of the wavelength components of light output from an optical amplification apparatus that is an object of control. The automatic gain control circuit produces a control signal used to adjust the strength of pumping performed in the optical amplification apparatus so that the maximum or minimum power will be set to a predetermined value.
An optical amplification apparatus in accordance with the present invention comprises a first optical amplifier and an optical amplifier control unit having either of the foregoing configurations for controlling the optical amplifier.
A transmission system in accordance with the present invention comprises an optical transmitter, an optical reproduction repeater, and an optical receiver. The optical transmitter transmits a light signal over a light transmission line. The optical reproduction repeater is inserted in the optical transmission line and amplifies the light signal traveling over the optical transmission line. The optical receiver receives the light signal sent over the optical transmission line. The optical reproduction repeater is realized with the optical amplification apparatus.